1. Field of the Invention
The present invention relates to an energization control apparatus for a glow plug used, for example, for pre-heating of a diesel engine.
2. Description of the Related Art
Conventionally, in an automobile, a glow plug having a heating resistor which generates heat upon supply of electric current is used so as to assist startup of an engine, or to stably operate the engine. A heating resistor whose resistance increases with its own temperature; i.e., a heating resistor having a positive correlation with its temperature, has been widely used for glow plugs. Furthermore, a constant power control scheme and a resistance control scheme have been known as schemes for controlling the supply of electric current to a glow plug including such a heating resistor.
In the constant power control scheme, electric power supplied to a glow plug is obtained from a voltage applied to the glow plug and a current flowing therethrough, and electric current is supplied to the glow plug such that electric energy obtained through integration of the electric power becomes equal to a predetermined amount of electric energy. According to this control method, the glow plug generates heat in accordance with the supplied electric energy. Therefore, the temperature of the glow plug can be increased to a predetermined temperature through supply of a predetermined amount of electric energy.
However, maintaining the glow plug at a constant temperature is difficult in the case where the glow plug is thermally influenced from the outside, such as the case where the heating resistor of the glow plug is cooled because of a disturbance caused by a change in engine speed, load (throttle opening), water temperature, etc. Maintaining a constant temperature of the glow plug requires obtaining information regarding the engine speed, load, etc., from, for example, an ECU, and controlling the effective voltage applied to the glow plug on the basis of the obtained information. However, in such a case, a problem of increased processing load may arise.
Meanwhile, in the resistance control scheme, the supply of electric current to a glow plug is controlled such that the resistance of the glow plug approaches a target resistance corresponding to a target temperature. According to the resistance control scheme, even in the case where the glow plug is influenced by a temperature change caused by a disturbance, the only requirement is to change an effective voltage to be applied in accordance with a change in resistance of the glow plug caused by the disturbance. Accordingly, unlike the above-described scheme, processing load does not increase, and the glow plug can be maintained at a constant temperature with relatively ease.
Incidentally, in order to control the supply of electric current to a glow plug in accordance with the resistance control scheme, a target resistance of the glow plug must be set. When the target resistance is set, deviation in resistance among individual glow plugs caused by various factors may be taken into consideration. Specifically, a resistance (pre-correction target resistance) which serves as a reference during control is set for each individual glow plug, and a target resistance is set on the basis of the pre-correction target resistance. Through performing control on the basis of the pre-correction target resistance set for each glow plug, energization control can be performed such that deviation among a plurality of glow plugs approaches zero.
Notably, the pre-correction target resistance is set as follows. In a state in which disturbances arising during operation of an internal combustion engine (e.g., cooling of the heating resistance by swirl or fuel injection) do not exist, the temperature of the glow plug is increased to a temperature which serves as a control target (target temperature), and the resistance of the glow plug at that time is obtained and set as a pre-correction target resistance. The target resistance can be obtained by correcting the obtained pre-correction target resistance in accordance with a change in water temperature or outside air temperature, a change in the target temperature, etc.
Also, the following method has been proposed so as to obtain the pre-correction target resistance (see, for example, Patent Document 1). That is, as described above, the resistance among individual glow plugs varies because of various factors. Therefore, even among glow plugs of the same model number, the relation between temperature and resistance may vary. However, the relation between a cumulative amount of supplied electric power and an amount of generated heat is determined by the material of the heating resistor of each glow plug, and its deviation is relatively small. In consideration of this point, electric current is first supplied to a glow plug serving as a reference such that its temperature rise becomes saturated at a target temperature, and a cumulative amount of electric power (cumulative electric energy) supplied to the glow plug at that time is obtained. The obtained cumulative electric energy is supplied to a glow plug when an internal combustion engine is not operated (stopped). Further, the resistance of the glow plug at that time (that is, when the resistance of the glow plug becomes saturated and the temperature of the glow plug becomes equal to the target temperature) is measured, whereby the pre-correction target resistance of the glow plug can be obtained. Notably, in the above-described technique, when electric power is supplied to a glow plug, a constant electric power is supplied to the glow plug.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2010-127487
3. Problems to be Solved by the Invention
In the above-described method, an operation of setting a pre-correction target resistance for an individual glow plug (hereinafter this operation will be referred to as “calibration”) is performed when an internal combustion engine is not operated, and, at the time of calibration, the glow plug is heated to a target temperature. That is, at the time of calibration, the glow plug may be heated to a high temperature in a state in which a driver and other persons are away from an automobile. Also, at the time of calibration, since the glow plug is heated to the target temperature, a large amount of electric power is consumed from a battery. From the viewpoints of safety and electric power consumption, the time required for calibration is preferably shortened as much as possible. However, in the case where constant electric power is supplied to a glow plug as in the above-described technique, a relatively long time may be needed to saturate the resistance of the glow plug, for the following reason.
That is, at the time of calibration, the resistance of the glow plug is obtained after the respective temperatures of not only the heating resistor but also a control coil, a center rod, and a power supply harness connected to the glow plug become saturated, and the resistance is set as a pre-correction target resistance. Accordingly, in the case where constant electric power is supplied to the glow plug, at an initial stage of electric power supply (a stage in which the temperatures of the control coil, the center rod, etc., have not yet become saturated), the ratio of the resistance of the heating resistor to the resistance of the glow plug (the sum of the resistance of the heating resistor and the resistances of the control coil, the center rod, the harness, etc.) is relatively large. Therefore, the heating resistor rapidly rises in temperature, and reaches a target temperature at a relatively early stage. However, the respective temperatures of the control coil, the center rod, etc., increase gradually because the temperature of the heating resistor is gradually transmitted to the control coil, the center rod, etc. Therefore, a long time is needed to saturate the resistance of the glow plug, and, as a result, a relatively long time is needed for calibration.